In chemical processing, such as plural component polyurethane processing, the proper mixing of the chemical components is essential to developing the final physical properties specified by the system supplier. In impingement designed mixing heads or spray guns, lowering the viscosities with heat helps to facilitate proper mixing. The two types of preheaters are typically utilized in impingement designed mixing heads/spray guns.
The first style, mass style, heats by conduction. Mass style heating utilizes a structural block, which is typically aluminum, into which holes are bored or small grooves cut and hydraulically connected to form a labyrinth through which the chemical passes. Heater rods are attached to or embedded in the block to raise the temperature of the surrounding structural mass, which in turn raises the temperature of the chemical within the holes/grooves. In this type of heating, the heater rods are isolated from the grooves or holes through which the chemical flows. Thus, heat is transferred from the heated mass to the chemical, which is either in a static or dynamic state within the chemical grooves, by means of conduction. The temperature of the mass, and, indirectly, the chemical, is maintained at the process temperature by means of a temperature controller and a sensor located within the mass. Typical mass style heating arrangements are disclosed, for example, in U.S. Pat. No. 2,866,885 to McIlrath, and U.S. Pat. No. 4,343,988 to Roller et al.
Mass style heaters have numerous advantages and disadvantages. Mass style heaters exhibit high thermal inertia in that, once at temperature, they tend to resist small temperature changes. As a result, mass style heaters generally provide stable temperature control if the chemical is maintained in a constant dynamic state or a constant static state. During the transition from the dynamic mode to the static mode, however, the mass ends to retain its temperature and pass it off to the static chemical causing an undesirable temperature spike. Conversely, as the chemical transitions from the static mode to the dynamic, the inefficiency of the mass heater causes a temperature drop at the outlet of the heater. Thus, mass style heaters are typically slow in responding to flow changes. Moreover, inasmuch as the labyrinth of drilled holes typically comprises relatively small grooves, it can develop backpressure during dynamic conditions.
The second style is the direct contact style heater. Direct contact style heaters utilize direct heating by placing heater rods into direct contact with the chemical. A heater rod is paced into a hydraulic tube of a given diameter. One or more such hydraulic tubes are typically connected to a manifold interconnecting other similarly configured tubes with an inlet and an outlet. The chemical traverses through the tubes in direct contact with the heater rods. Examples of direct contact style heaters are shown, for example, in U.S. Pat. No. 4,465,922 to Kolibas.
As with the mass style heater, direct contact style heating has both its advantages and disadvantages. Because there is little thermal inertia, direct contact style heating responds well to flow changes. Additionally, such heaters come to temperature quickly, providing a very fast warm up cycle. Direct style heaters provide more efficient heat transfer than mass style heaters. Direct style heaters provide a much greater difference in temperature between the set point temperature and the fire rod surface temperature such that the temperature control is less stable in steady conditions than mass style heaters. Further, direct contact heaters have historically been more costly to manufacture and assemble than mass style heaters. Moreover, the physical dimensions of direct style heaters constrain the number of tubes, thus shortening the contact surface area available for heat transfer.
Accordingly, there exists a need for a heating arrangement that provides the advantages of the currently available heaters, while minimizing or eliminating the disadvantages of the same. The invention provides such an arrangement. The advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.